1. Field of the Invention
The present invention relates to a brake pressure control valve located in the outlet port of a master cylinder for controlling the brake pressure applied to the rear wheels of a motor vehicle, and more particularly to a brake pressure control valve adapted for use with a double piping type hydraulic braking system.
2. Description of the Prior Art
Various kinds of brake pressure control valves are known which control the brake pressure delivered to the rear wheels of the vehicle so that once the master cylinder pressure reaches a certain value (critical pressure), the brake pressure for the rear wheels thereafter increases at a lesser rate than the input pressure from the master cylinder. With this function, the dangerous skid phenomenon wherein the rear wheels are locked prior to the front wheels under braking of the vehicle is prevented.
Now, in order to assure safety of the vehicle, a so-called double piping type hydraulic braking sytem is widely employed nowadays, which includes two groups of braking systems so that even when one system fails to operate, the other system operates to brake the vehicle. X-piping system and J--J piping system are known as the double piping system.
When the double piping system is employed, two brake pressure control valves are necessary which are respectively mounted in the two (double) hydraulic braking system. Usually, these two control valves are assembled in a single housing to reduce mounting space and production cost. Furthermore, for such purpose, it is known to use a housing integrated with a tandem master cylinder.
However, most of the brake pressure control valves are constructed to employ the housing as an essential part thereof, so that operation of each control valve can not be expected until the valve is completely assembled in the housing. In fact, hitherto, the performance test of each valve has been carried out after assembling these two valves in the housing. Thus, when a defect is found in the assembled valve unit, it is necessary to take out the parts of the valves from the housing one by one in awkward manner. The disassembling work and next-necessitated reassembling work are quite troublesome and thus take much time.
One of the brake pressure control valves which are free of the above-mentioned drawbacks is disclosed in Japanese Patent Application Nos. 56-186220 & 56-186221. The valve disclosed therein comprises generally a housing integrated with a master cylinder, and two identical valve units coaxially and symmetrically arranged in the housing, wherein each of the valve units comprises a casing in which substantially all essential parts of the valve are housed, and a free piston means removably mounted to the casing in a manner to retain the essential parts in the casing. That is, each valve unit of the valve constitutes a finished device without using any parts of the housing. Thus, each valve unit can be independently pre-tested before being assembled in the housing. Even if trouble is found in the valve unit in the housing, the repair work requires only replacing the valve unit in trouble with new one, which is quite easy.
However, the improved valve as stated hereinabove still has a weak point in self-centering characteristics of the free pistons thereof which contribute to a so-called fail-safe function of the valve. In fact, when the brake pedal is rapidly released, there sometimes appears a pressure difference between the input pressures of the paired valve units causing displacement of the free pistons from their home (or neutral) positions. However, in the valve as mentioned above, rapid self-centering movements of the free pistons is not assured because of its inherent construction. The rapid self-centering is quite important because if the brake pedal is depressed with the free pistons being not centered, the brake pressures applied to the rear wheels become unbalanced thereby causing unstable braking of the vehicle.